1. Field of the Invention
The present invention relates to a battery voltage detection device suitably used for measuring a low impedance voltage which is not earthed; for example, the voltage of a battery mounted as a power supply for a motor on an electric vehicle, hybrid electric vehicle, or the like.
2. Description of the Related Art
In an electric vehicle, a hybrid electric vehicle, or the like, a motor is mounted as a power source. A battery is used as an electric power supply for the motor. Such a battery requires a high voltage and a high power output. Therefore, a battery pack including a plurality of serially-connected battery cells is used as the battery for driving the motor. Usually, rechargeable secondary batteries are used as the battery cells. Since a battery pack used in an electric vehicle requires a high voltage, the battery pack is mounted on a vehicle so as to be isolated from the chassis earth for safety reasons.
In the electric vehicle, a battery voltage detection device is provided for monitoring the occurrence of malfunctions in the battery pack. The battery voltage detection device detects the voltage of each battery block which includes a plurality of battery cells as one unit. FIG. 3 shows an exemplary structure of such a battery voltage detection device.
In FIG. 3, a battery pack 11 includes a plurality of battery blocks 11a. A plurality of voltage detection terminals 12 are provided between the battery blocks 11a. Each voltage detection terminal 12 is connected to a respective one of a first plurality of FETs (field-effect transistors) 43, which is a first switching element. Each of the first plurality of FETs 43 forms an SSR (solid-state relay). Some of the first plurality of FETs 43, which are connected to the odd-numbered voltage detection terminals 12 in the battery pack 11, are together connected to one terminal of a capacitor 46 and to a second FET 45a which is a second switching element. The remainder of the first plurality of FETs 43, which are connected to the even-numbered voltage detection terminals 12 in the battery pack 11, are together connected to the other terminal of the capacitor 46 and to a third FET 45b which is a third switching element.
The second FET 45a and the third FET 45b are connected to respective input terminals of a differential amplifier 20. The differential amplifier 20 includes a first operational amplifier 21. The second FET 45a is connected to a positive input terminal of the first operational amplifier 21 via a first resistor 22a. The third FET 45b is connected to a negative input terminal of the first operational amplifier 21 via a second resistor 22b. The positive input terminal of the first operational amplifier 21 receives, via a third resistor 22c, the output of a second operational amplifier 23 which generates a reference voltage. The output of the first operational amplifier 21 is fed back to the negative terminal of the first operational amplifier 21 via a fourth resistor 22d. The output of the first operational amplifier 21 is output to an A/D converter 30 as the output of the differential amplifier 20.
A voltage dividing circuit 24 is a series circuit formed by resistors 24a and 24b. The potential of the connection point of the resistors 24a and 24b is input to the positive input terminal of the second operational amplifier 23. The output of the second operational amplifier 23 is fed back to the negative input terminal of the second operational amplifier 23.
In a battery voltage detection device having such a structure, the voltages of the battery blocks 11a in the battery pack 11 are detected in turn by the differential amplifier 20.
In the first step of the voltage detection process, the second FET 45a and the third FET 45b connected to the differential amplifier 20 are turned off. Then, a first FET of the first plurality of FETs 43, which is connected to the first voltage detection terminal 12 in the battery pack 11, and a second FET of the first plurality of FETs 43, which is connected to the second voltage detection terminal 12 in the battery pack 11, are turned on. Thus, the first battery block 11a is connected to the capacitor 46, and charges the capacitor 46.
After the capacitor 46 has been charged, the pair of the FETs 43 are turned off, and then the second FET 45a and the third FET 45b are turned on. Thus, the voltage of the capacitor 46 is applied to the differential amplifier 20.
In the differential amplifier 20, a reference voltage of 2.5 V is applied from the second operational amplifier 23 to the positive input terminal of the first operational amplifier 21 via the third resistor 22c. Therefore, the voltage of the capacitor 46, which has been input to the differential amplifier 20, is detected within a range up to 5 V from a reference of 2.5 V.
Similarly, while the second FET 45a and the third FET 45b are off, the first FETs 43 connected to the second voltage detection terminal 12 and the third voltage detection terminal 12 are turned on, so that the capacitor 46 is charged with the second battery block 11a. Then, the first FETs 43 are turned off, and the second FET 45a, and the third FET 45b are turned on. Thus, the voltage of the second battery block 11a in the battery pack 11 is detected.
In this case, the polarity of the voltage which has been input to the first operational amplifier 21 of the differential amplifier 20 is opposite to that of the first battery block 11a. Therefore, the first operational amplifier 21 detects the voltage of the second battery block 11a within a range down to 0 V from a reference of 2.5 V.
Subsequently, the voltages of the other battery blocks 11a are detected in turn in a similar manner.
Although the voltages having the opposite polarities are input in turn from the battery blocks 11a forming the battery pack 11 to the differential amplifier 20, the voltages are detected without switching the polarity of the reference potential of the first operational amplifier 21. The detected voltages of the battery blocks 11a are input to the A/D converter 30. In the A/D converter 30, input voltages are A/D converted, and then output to a computing unit, such as a CPU.
However, in such a battery voltage detection device, each of the first plurality of FETs 43, which forms an SSR, has an inter-terminal capacitance. This inter-terminal capacitance may cause problems in the detection of the voltages of the battery blocks 11a. Specifically, in order to detect the voltage of one of the battery blocks 11a, a pair of first plurality of FETs 43 connected to voltage detection terminals 12 of this battery block 11a are turned on, then the capacitor 46 obtains a voltage value substantially equal to that of the connected battery block 11a. However, when each of the first plurality of FETs 43 is turned off, each of the first plurality of FETs 43 is charged with an electric charge of the capacitor 46, because each of the first plurality of FETs 43 has inter-terminal capacitance. Therefore, the voltage value of the capacitor 46 may vary. As a result, the voltages of the battery blocks 11a may not be detected with high precision.
Furthermore, when the number of the battery blocks 11a in the battery pack 11 is an even number, the number of the first plurality of FETs 43 connected to respective terminals of the capacitor 46 are different. In the case of detecting a voltage of an even-numbered battery block 11a, the number of the first plurality of FETs 43 connected to the capacitor 46 is increased by one, in comparison to the number of the first plurality of FETs 43 in the case of detecting a voltage of an odd-numbered battery block 11a. Thus, there is a difference in the inter-terminal capacitances connected to the capacitor 46 between the case of detecting the voltage of the odd-numbered battery block 11a and the case of detecting the voltage of the even-numbered battery block 11a. As a result, the voltages of the battery blocks 11a may not be detected with a high precision.
In this case, by making the capacitance of the capacitor 46 sufficiently larger than those of the first plurality of FETs 43, variation in the voltage of the capacitor 46 can be suppressed. However, as the capacitance increases, the capacitor 46 becomes more expensive. Thus, cost-effectiveness is reduced in such a case. Furthermore, as the number of the battery blocks 11a increases, the number of lines connected to the capacitor 46 increases, and thus, the number of first plurality of FETs 43 connected in parallel to the capacitor 46 increases. Thus, since the total capacitance connected to the capacitor 46 increases, the capacitance of the capacitor 46 effectively increases and the variation in the voltage of the capacitor 46 may not be suppressed.
Further still, as described above, a battery pack 11 used in an electric vehicle is mounted so as to be isolated from the chassis earth. That is, the battery pack 11 is connected to the chassis earth with a large impedance. The output of the battery pack 11 varies with respect to the chassis earth depending upon the magnitude of the load on the battery pack 11, and as a result, a common mode noise may be generated. This common mode noise affects the inter-terminal capacitances of the first plurality of FETs 43, and may affect the voltage to be charged in the capacitor 46. Thus, the precision in detection of the voltages of the battery blocks 11a decreases.
In order to prevent such influences caused by the common mode noise, the differential amplifier 20 may be used for controlling the output of the battery pack 11. However, in this case, the battery pack installed so as to be isolated from the chassis earth requires an isolated-type, DC-to-DC differential amplifier. Therefore, the number of components is increased, the circuit arrangement becomes complicated, and the cost-effectiveness is reduced.
When the amplifier is provided in the battery pack 11, the first plurality of FETs 43 having different inter-terminal capacitances are connected between the differential amplifier and the capacitor 46. Therefore, due to the different impedances of each of the first plurality of FETs 43, a new common mode noise may be generated, and the voltages of the battery blocks 11a may not be detected with a high precision.
In the differential amplifier 20 to which the voltage of the capacitor 46 is input, the gain can be changed by varying resistance values of the first resistor 22a to the fourth resistor 22d, and the offset can be changed by varying the reference potential of the first operational amplifier 21. Therefore, such a differential amplifier 20 is suitable for measuring a low impedance analog voltage which has an unfixed potential. Furthermore, the common mode noise can be suppressed in an increased proportion.
However, the characteristics of the operational amplifiers and the resistors, which form the differential amplifier 20, may vary due to variation in temperature or deterioration with the passage of time. In view of such circumstances, operational amplifiers or resistors having characteristics more resistant to temperature variation may be used. However, such operational amplifiers and resistors are expensive and reduce cost-effectiveness.
According to one aspect of the present invention, there is provided a battery voltage detection device for detecting voltages of battery blocks in a battery pack including a plurality of N battery blocks connected in series, comprising: a plurality of (N+1) voltage detection terminals connected to the plurality of N battery blocks; a first plurality of switches each having an inter-terminal capacitance, the plurality of switches being connected to the respective voltage detection terminals connected to the battery blocks; a second switch having an inter-terminal capacitance, to which the first plurality of switches are collectively connected, the first plurality of switches being connected to odd-numbered voltage detection terminals; a third switch having an inter-terminal capacitance, to which the first plurality of switches are collectively connected, the first plurality of switches being connected to even-numbered voltage detection terminals; a pair of fourth switches connected in series to the second switch and the third switch; a capacitor provided between the connection point of the second switch and one of the fourth switches, and the connection point of the third switch and the other of the fourth switches; and a differential amplifier having input terminals to which the fourth switches are connected.
In one embodiment of the present invention, a battery voltage detection device further comprises: an A/D converter for converting the output voltage of the differential amplifier to digital value; and a computing device for processing the digital value converted by the A/D converter.
In one embodiment of the present invention, the differential amplifier uses a voltage obtained from a voltage dividing circuit as the reference voltage.
In one embodiment of the present invention, the voltage obtained from the voltage dividing circuit is output as an output of the differential amplifier.
In one embodiment of the present invention a battery voltage detection device further comprises: an A/D converter for measuring the voltage obtained from the voltage dividing circuit.
In one embodiment of the present invention, the differential amplifier has a voltage dividing circuit formed by a resistor having the same resistor value as that of a resistor which forms a gain of the differential amplifier; and the battery voltage detection device further includes an A/D converter for measuring the output of the voltage dividing circuit.
Thus, the invention described herein makes possible the advantages of providing a battery voltage detection device which is capable of detecting battery voltage with high precision and which does not reduce the cost-effectiveness.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.